Syntax

A JavaScript array is initialized with the given elements, except in the case where a single argument is passed to the Array constructor and that argument is a number. (See below.) Note that this special case only applies to JavaScript arrays created with the Array constructor, not with array literals created with the bracket syntax.

arrayLength

If the only argument passed to the Array constructor is an integer between 0 and 232-1 (inclusive), a new, empty JavaScript array and its length is set to that number. If the argument is any other number, a RangeError exception is thrown.

Description

Arrays are list-like objects that come with a several built-in methods to perform traversal and mutation operations. Neither the size of a JavaScript array nor the types of its elements are fixed. Since an array's size can grow or shrink at any time, JavaScript arrays are not guaranteed to be dense. In general, these are convenient characteristics, but if these features are not desirable for your particular use case, you might consider using WebGL typed arrays.

Accessing array elements

JavaScript arrays are zero-indexed; the first element of an array is actually at index 0, and the last element is at the index one less than the value of the array's length property.

var arr = ["this is the first element", "this is the second element"];
console.log(arr[0]); // prints "this is the first element"
console.log(arr[1]); // prints "this is the second element"
console.log(arr[arr.length - 1]); // prints "this is the second element"

Array elements are just object properties, in the way that toString is a property. However, note that trying to access the first element of an array as follows will throw a syntax error:

console.log(arr.0);

Note that there is nothing unique about JavaScript arrays and their properties that causes this. JavaScript properties that begin with a digit cannot be referenced with the dot notation. They must be accessed using bracket notation. For example, if you had an object with a property "3d", it would not be possible to access it using dot notation, either. It, too, would have to be referenced using bracket notation. This similarity is exhibited in the following two code samples:

Note that in the 3d example, "3d" had to be quoted. It's possible to quote the JavaScript array indexes as well (e.g., years["2"] instead of years[2]), though it's not necessary. The 2 in years[2] eventually gets coerced into a string by the JavaScript engine anyway, through an implicit toString conversion. It is for this reason that "2" and "02" would refer to two different slots on the years object and the following example logs true:

console.log(years["2"] != years["02"]);

Relationship between length and numerical properties

A JavaScript array's length property and numerical properties are connected. Several of the built-in array methods (e.g., join, slice, indexOf, etc.) take into account the value of an array's length property when they're called. Other methods (e.g., push, splice, etc.) also result in updates to an array's length property.

When setting a property on a JavaScript array when the property is a valid array index and that index is outside the current bounds of the array, the array will grow to a size large enough to accommodate an element at that index, and the engine will update the array's length property accordingly:

Creating an array using the result of a match

The result of a match between a regular expression and a string can create a JavaScript array. This array has properties and elements that provide information about the match. An array is the return value of RegExp.exec, String.match, and String.replace. To help explain these properties and elements, look at the following example and then refer to the table below:

// Match one d followed by one or more b's followed by one d
// Remember matched b's and the following d
// Ignore case
var myRe = /d(b+)(d)/i;
var myArray = myRe.exec("cdbBdbsbz");

The properties and elements returned from this match are as follows:

Property/Element

Description

Example

input

A read-only property that reflects the original string against which the regular expression was matched.

cdbBdbsbz

index

A read-only property that is the zero-based index of the match in the string.

1

{{ mediawiki.external(0) }}

A read-only element that specifies the last matched characters.

dbBd

{{ mediawiki.external(1) }}, ...{{ mediawiki.external('n') }}

Read-only elements that specify the parenthesized substring matches, if included in the regular expression. The number of possible parenthesized substrings is unlimited.